Moving individual tests into subdirectory.

libfuzzer_main is also used in environments where sample fuzzer
dependencies do not exists (e.g. webrtc build).

BUG=webrtc:4771
BUG=539572
TBR=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1458883008

Cr-Commit-Position: refs/heads/master@{#360782}

diff --git a/BUILD.gn b/BUILD.gn
index 0c36d83b..4fd53cc 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -705,7 +705,11 @@
   }
 
   if (use_libfuzzer) {
-    deps += [ "//testing/libfuzzer:libfuzzer_main" ]
+    # these are needed only for gn to discover build files.
+    deps += [
+      "//testing/libfuzzer:libfuzzer_main",
+      "//testing/libfuzzer/fuzzers:string_to_int_fuzzer"
+    ]
   }
 }